1. Field of the Invention
The invention relates to an optical measuring apparatus for optical, for example interferometric, measuring of an optical imaging system, which is intended for imaging of a useful pattern, and to a method for operation, including imaging error correction, of such optical imaging system.
2. Description of the Related Art
The imaging quality of optical imaging systems is subject to requirements which are becoming ever more stringent. One example of this is projection objectives for microlithographic production of semiconductor components and other finely structured components, which structures should be largely free of imaging errors in the submicrometer range. Since, because of the complex optical design, it is generally impossible to derive the optical characteristics of the objectives from theoretical calculations, reliable measurement of the optical characteristics is necessary before, and in some cases also even during, operation at the point of use, for example in a so-called wafer scanner for wafer exposure.
Interferometric measurement methods are frequently used for this purpose. An apparatus which operates in the manner of a Shearing interferometer for wavefront detection and which allows rapid, high-precision measurement of high-resolution photolithographic projection objectives, is described in the Laid-Open publication DE 101 09 929 A1. Measuring devices such as these typically contain a wavefront source on the object side, that is to say on the object side of the optical imaging system to be measured, in order to produce at least one radiation wavefront, which passes through the imaging system, and a diffraction grating on the image side, that is to say on the image side of the optical imaging system to be measured, as well as a position-resolving detector, which is arranged downstream from this diffraction grating, for provision and detection of interferometric information which is indicative of imaging errors in the system to be measured. The imaging errors can be determined from the detected interference information by suitable evaluation means.
By way of example, a so-called source or wavefront module can be used as an illumination module as the wavefront source, which is connected to an illumination section and has a measurement mask in the form of a so-called hole or coherent mask, which has a suitable measurement pattern. If required, the illumination part may correspond to that which is associated with the optical imaging system at its intended point of use, for example in a microlithography system. In this case, the measuring device can be integrated in the microlithography system, and all that is necessary in each case to carry out a measurement is just to replace the useful mask, to which the useful pattern is applied, by the illumination module, and to introduce the detector instead of a wafer.
Furthermore, the prior German Patent Application 102 17 242.0 proposes the capability to measure the optical imaging system by means of Shearing interferometry during its normal operation, for which purpose a mask is provided for the wavefront source, on which mask a measurement pattern is formed in addition to a useful pattern. The interference information which is obtained in this way during normal imaging operation of the optical imaging system, and which is indicative of aberrations in the imaging system, is evaluated by an evaluation unit and is used for correction of the aberrations that are found, for which purpose an appropriate aberration closed-loop or open-loop control system is provided.
In addition to such interferometric measurement methods, non-interferometric measurement methods are also used for aberration determination, for example on projection objectives of microlithographic projection exposure systems, such as the Shack-Hartmann method.
As the imaging quality requirements become more stringent, particularly for projection objectives in microlithography, imaging errors or aberrations resulting from the so-called heating effect or “lens-heating” effect are no longer intrinsically negligible, with these expressions meaning a change in the imaging characteristics of the microlithography projection objective or of some other optical imaging system resulting from interaction between the radiation that is used and the objective and/or those system components which are active for imaging purposes. One difficulty in the determination of these heating-dependent imaging errors is that this effect often decays very quickly when the irradiation is ended. Thus, if the system components which are active for imaging purposes are first of all irradiated intensively, for example with a radiation intensity which is comparable to that during normal operation, and the irradiation is then stopped in order subsequently to measure the heating-dependent imaging errors, these measurement results represent the imaging errors that occur during normal irradiation operation only to a very restricted extent.
It is an object of the invention to provide a measuring apparatus of the type mentioned initially as well as a method for operation of an optical imaging system, including imaging error correction, whose imaging errors can be determined using such measuring apparatus, by means of which the heating-dependent imaging errors can be determined comparatively easily and reliably, and can be taken into account for correction purposes.